Director James Gunn is currently writing Guardians of the Galaxy 3 and tells IGN while talking about the new Disney ride that the third movie will lead to other movies:

We talk about it all the time. It’s already a part of what I’m doing with Marvel and the conversations that Kevin and I have every day. When we’re talking about Guardians of the Galaxy 3, and that story, that’s a story that will lead to other stories. It’s innate to the DNA that that’s the end of a trilogy but it’s the beginning of a whole other element of the Marvel Cosmic universe. That’s a part of what I’m doing now.

Another possibility could be a Richard Rider Nova movie (and bring Wendell Vaughn Quasar along for the ride!) I know a Richard Rider Nova movie is on the top of every Marvel Cosmic fan's list, and I also know a lot of Nova fans have been bugging James Gunn about it (myself included - Gunn calls me out on Facebook Live). The problem James Gunn apparently has with Richard Rider Nova (Gunn cut him from the first movie) is that Richard Rider is human--Gunn wants Star-Lord to be the only human character in space. So perhaps with Guardians of the Galaxy 3 being the last time we'll see the current team together, maybe James Gunn will lift his mandate on Star-Lord being the only human.

I know Gunn has also said he would like to use Annihilus, presently owned by Fox Studios, so hopefully one day a deal can be made where Marvel gets the rights back, as some sort of adaptation of Annihilation would rock!